Introduction
The respiratory system consists of a group of passages that remove particles from incoming air and transport it from outside the body into and out of the lungs. It also includes numerous microscopic air chambers in which gas exchanges take place between the air and the blood. The entire process of exchanging gases between the atmosphere and the body cells is called respiration, and it involves several events. These include the movement of air in and out of the lungs - commonly called breathing, or pulmonary ventilation; the exchange of gases between the air in the lungs and the blood; the transport of gases by the blood between the blood and body cells. The utilization of oxygen and production of carbon dioxide by the cells is called cellular respiration.

Nose The nose is supported internally by bone and cartilage. Its two nostrils, called the external nares, provide opening through which air can enter and leave the nasal cavity. These openings are guarded by numerous internal hairs, which help prevent the entrance of relatively large particles sometimes carried in the air. Nasal Cavity The nasal cavity, a hollow space behind the nose, is divided medially into right and left portions by the nasal septum, which is composed of bone and cartilage. Nasal conchae curl out from the lateral walls of the nasal cavity on each side, dividing the cavity into passageways. They support the mucous membrane that lines the nasal cavity and help increase its surface area. The mucous membrane contains pseudostratified ciliated epithelium that is rich in mucus-secreting goblet cells.

blood vessels, and as air passes over the membrane, heat leaves the

blood and warms the air. In this way, the temperature of the incoming

air quickly adjusts to that of the body. In addition, the incoming

air tends to become moistened by the evaporation of water from the

mucous lining. The sticky mucus secreted by the mucous membrane

entraps dust and other small particles entering with the air.

As the cilia of the epithelial lining move, a thin

layer of mucus and entrapped particles are pushed toward the pharynx.

When the mucus reaches the pharynx, it is usually swallowed. In the

stomach, any microorganisms in the mucus are likely to be destroyed

by the action of gastric juices.

Paranasal Sinuses

The paranasal sinuses are air-filled spaces located within (and named from) the

maxillary , frontal, ethmoid, and sphenoid bones of the skull. These

spaces open into the nasal cavity and are lined with mucous membranes

that are continuous with the lining of the nasal cavity.

Although the paranasal sinuses function mainly to

reduce the weight of the skull, they also serve as resonant chambers,

which affect the quality of the voice.

Pharynx

The pharynx

(throat) is located behind the oral cavity and between the nasal

cavity and larynx. It functions as a passageway for food traveling

from the oral cavity to the esophagus and for air passing between the

nasal cavity and larynx. It also aids in producing the sounds of

speech. The pharynx consists of three subdivisions: nasopharynx (near nasal cavity),

oropharynx (back

of throat, in mouth), and laryngopharynx (near the

larnyx).

Larynx

The larynx is an

enlargement in the airway at the top of the trachea and below the

pharynx. It serves as a passageway for air moving in and out of the

trachea and functions to prevent foreign objects from entering the

trachea. In addition, it houses the vocal

cords.

The larynx is composed primarily of muscles and cartilages, which form the framework of the larynx and are bound together by elastic tissue. The largest of the cartilages are the thyroid, cricoid, and epiglottic cartilages. Inside the larynx, two pair's of horizontal folds, composed of muscle tissue and connective tissue with a covering of mucous membrane, extend inward from the lateral walls. The upper folds are called false vocal cords because they do not function in the production of sounds. The muscle fibers within these folds help to close the airway during swallowing. The lower folds are the true vocal cords. They contain muscle tissue and elastic fibers and are responsible for vocal sounds,

which are created when air is forced between

the vocal cords, causing vibrations in the air column above them.

This action generates sound waves, which can be formed into words by

changing the shape of the pharynx and oral cavity and by using the

tongue and lips.

The quality of a vocal sound can be altered by

changing structures within the larynx, pharynx, and oral cavity.

Thus, the pitch (musical tone) of a sound is controlled by

contracting or relaxing muscles that alter the tension on the vocal

folds. Increasing tension produces a higher pitch, and decreasing

tension causes a lower pitch. The intensity (loudness) of a sound is

related to the force of the air passing through the vocal folds.

Louder sound is produced by stronger blasts of air; softer sound is

produced by weaker blasts of air.

During normal breathing, th vocal cords remain

relaxed, and the opening between them, called the glottis, appears as a triangular

slit. When food or liquid is swallowed, however, the glottis is

closed by muscles within the false vocal cords, and this prevents the

food or liquid from entering the trachea.

The epiglottic cartilage suports a flaplike

structure called the epiglottis. This structure

usually stands upright and allows air to tenter the larynx. During

swallowing, however, the larynx is raised, and the epiglottis is

pressed downward. As a result, the epiglottis partially covers the

opening into the larynx and helps to prevent foods and liquids from

entering the air passages.

Trachea

The trachea

(windpipe) is a flexible cylindrical tube about 2.5 cm in diameter

and 12.5 cm in length. It extends downward in front of the esophagus

and into the throacic cavity, where it splits into right and left

bronchi.

The inner wall of the trachea is lined with a ciliated mucous membrane that contains many goblet cells. As mentioned before, this membrane continues to remove particles from the incoming air and to move entrapped particles upward into the pharynx. Within the tracehal wall are about twenty C-shaped peices of hyaline cartilage, arranged one above the other.

The open ends of these incomplete rings are

directily posteriorly, and the gaps between their ends are filled

with smooth muscle and connective tissues. These cartilaginous rings

prevent the trachea from collapsing and blocking the airway. At the

same time, the soft tissues that complete the rings in the back allow

the nearby esophagus to expand as food moves throught it on the way

to the stomach.

Bronchial Tree

The bronchial tree consists of branched airways leading from the trachea to

the microscope air sacs in the lungs. It begins with the right and

left primary bronchi,which arise from the trachia at the level of the fifth

thoracic vertebra.

A short distance from its origin, each primary bronchus divides into secondary bronchi, which in turn branch again and again into finer and finer tubes. Amoung theses smaller tubes are some called bronchioles. They continue to devide, giving rise to very thin tubes called alveolar ducts. These ducts terminate in groups of microscopic chambers called alveoli, which are surrounded by capillary nets.

Although the structure of the bronchus is

similar to that of the trachea, as finer and finer branches are given

off, the amount of cartilage in the walls decreases and finally

disappears in the bronchioles. As cartilage decreases, however, a

layer of smooth muscle surrounding the tube becomes more prominenet.

This muscular layer remains even in the smallest bronchioles, but

only a few muscle fibers occur in the alveolar ducts.

The branches of the bronchiole tree serve as air

passages, which continue to remove particles from the incoming air

and distribute it to the alveoli in all parts of the lungs. The

alveoli, in turn, provide a large surface area of thin

squamous epithelial cells through which gas exchanges can easily occur. During these

exchanges, oxygen diffuses through alveolar walls and enters the

blood in nearby capillaries, and carbon dioxide diffuses from the

blood through the walls and enters the alveoli.

It is estimated that there are about 300 million

alveoli in an adult lung and that these spaces have a total surface

area between 70 and 80 square meters.

Lungs

The lungs are

soft, spongy, cone-shaped organs located in the thoracic cavity. The

right and left lungs are seperated medially by the heart and

mediastinum, and they are enclosed by the diaphragm and thoracic

cage.

Each lung occupies most of the thoracic space on

its side and is suspended in the cavity by its attachments, which

include a bronchus and some large blood vessels. These tubular parts

are connected to the lung on its medial surface. A layer of serous

membrane, the visceral

pleura, is firmly attached to the surface

on each lung, and this membrane folds back to become the

parietal pleura.

The parietal pleura, in turn, forms part of the mediastinum and lines

the inner wall of the thoracic cavity.

The potential space between the visceral and parietal pleurae is called the pleural cavity, and it conains a thin film of serous fluid. This fluid lubricated the adjacent pleural surfaces, reducing friction as they move against each other during breating. It also helps hold the pleural membranes together, as explained later. The right lung is larger than the left one and is divided into three lobes. The left lung consists of two lobes. Each lobe is supplied by a major branch of the bronchial tree. A lobe also has connections to blood and lymphatic vessels and is enclosed by connective tissue. Thus, the substance of a lung includes air passages, alveoli, blood vessels, connective tissue, lymphatic vessels, and nerves.